[c-nsp] Strict multihoming supported on a Cisco?
Marcel Lammerse
lammerse at xs4all.nl
Wed Jul 28 16:52:25 EDT 2004
Hi all,
an incoming ip packet, addressed to a Cisco router, will be forwarded
to the destination address even if that address is not the ip address
of the directly connected interface:
spectrum2#sh ip int brief
Interface IP-Address OK? Method Status Protocol
Ethernet0 192.168.2.1 YES NVRAM up
up
Serial0 unassigned YES NVRAM up
up
Serial0.1 172.16.1.1 YES NVRAM up
up
Serial0.100 212.189.28.2 YES NVRAM up
up
Serial1 unassigned YES NVRAM administratively down
down
spectrum2#
Testmachine has an ip of 192.168.2.2 and has its default gateway
pointing to 192.168.2.1
[test at testmachine]$ ping 212.189.28.2
PING 212.189.28.2 (212.189.28.2) from 192.168.2.2 : 56(84) bytes of
data.
64 bytes from 212.189.28.2: icmp_seq=1 ttl=255 time=6.20 ms
64 bytes from 212.189.28.2: icmp_seq=2 ttl=255 time=2.08 ms
64 bytes from 212.189.28.2: icmp_seq=3 ttl=255 time=2.09 ms
I've heard some security concerns about this. Is there a way of
enforcing what is known as the Strong End-System Model (RFC1122) or
strict multihoming behavior on a Cisco router? Or would that break
routing functionality (and thus would explain why I haven't seen it
anywhere in the manuals)?
From the rfc:
There are two key requirement issues related to multihoming:
(A) A host MAY silently discard an incoming datagram whose
destination address does not correspond to the physical
interface through which it is received.
(B) A host MAY restrict itself to sending (non-source-
routed) IP datagrams only through the physical
interface that corresponds to the IP source address of
the datagrams.
DISCUSSION:
Internet host implementors have used two different
conceptual models for multihoming, briefly summarized
in the following discussion. This document takes no
stand on which model is preferred; each seems to have a
place. This ambivalence is reflected in the issues (A)
and (B) being optional.
o Strong ES Model
The Strong ES (End System, i.e., host) model
emphasizes the host/gateway (ES/IS) distinction,
and would therefore substitute MUST for MAY in
issues (A) and (B) above. It tends to model a
multihomed host as a set of logical hosts within
the same physical host.
o Weak ES Model
This view de-emphasizes the ES/IS distinction, and
would therefore substitute MUST NOT for MAY in
issues (A) and (B). This model may be the more
natural one for hosts that wiretap gateway routing
protocols, and is necessary for hosts that have
embedded gateway functionality.
Regards,
Marcel
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